Conventional radionuclide emission tomography cameras construct three-dimensional images of an object's radionuclide distribution from a sequence of two-dimensional images collected through a single collimator or number of collimator segments from a large number of viewing angles around the object being imaged. The very nature of the image reconstruction process, that is, the underlying mathematical theory and associated computer reconstruction algorithms, is such that in a preferred construction a tomographic field of view in the camera system encompasses the entire object being imaged. A tomographic field of view is the enclosed region of a field defined by the intersections of one or more collimator fields of view as the collimators rotate about the object of view through 2.pi. radians. Failure to include the entire imaged object in a tomographic field of view results in insufficient information to uniquely reconstruct its three-dimensional radionuclide distribution.
When a rotating planar radionuclide camera is employed to collect the images for reconstruction, a single continuous parallel hole (channel) or converging hole collimator, having a field of view encompassing the object, is used in order to restrict emissions received by the camera detector from the object to those gamma rays following parallel or diverging projections toward the detector. In the case of stationary annular camera detector with rotating collimator, such as in the patents of Hattori et al., U.S. Pat. No. 4,389,569, and Genna et al., U.S. Pat. No. 4,584,478, a rotating annular collimator system is segmented into a multiplicity of either parallel hole or converging hole collimators; however, each of these collimator segments still has a tomographic field of view as large as, or larger, than the imaged object, and provides only one such field of view.
A shortcoming of both of these systems is that the imaged object's radionuclide distribution is sampled either with uniform efficiency in the case of the parallel hole collimator or, in the case of the annular camera with rotating converging hole collimator, the center of the tomographic field of view is sampled with a lower efficiency than the periphery. Experimental studies of the effect of uniform sampling (Pang, S. C. and Genna, S., "Noise Propagation in 3-D Fourier Convolution Reconstruction" in Image Processing for 2-D and 3-D Reconstruction from Projections, Optical Society of America, PD-11, 1975) using a uniformly emitting water phantom have shown a substantial increase in the variance per pixel or decrease in the signal-to-noise ratio of the reconstructed data near the central portion of the imaged phantom. In clinical applications, however, the central regions of an imaged human body part are typically those in which enhanced imaging ability is desired, i.e., less variance in the measured data.